1. Field of the Invention
This invention relates to gas separation and purification, and more particularly, to flue gas treatment for removal of polluting constituents.
2. Description of Prior Art
The reduction of air pollution by flue gases has recently been of particular interest because of the detrimental impact of components of these emissions on air quality. In the consumption of lignite-based fuels, particularly in power plants, the two major problematic constituents have been SO.sub.2 gas and flyash. There have been numerous liquid scrubbing systems taught in the prior art in efforts to cleanse the flue gases generated by such power systems. In such scrubbing systems, removal and/or neutralization of the SO.sub.2 in the liquid scrubbing agent has been sought to prevent another environmental problem, namely water pollution.
Scrubber systems as taught in the prior art which teach treatment of the scrubber discharge include:
U.s. pat. No. 1,271,899 to Howard et al. which teaches water scrubbing of furnace gases followed by alkali treatment of the water; PA1 U.s. pat. No. 3,520,649 to Tomany et al. which teaches scrubbing of furnace gases wherein an alkali is added to the scrubbing slurry to neutralize SO.sub.2 ; and PA1 U.s. pat. No. 3,708,266 to Gustavsson which teaches an apparatus for cleansing flue gases containing SO.sub.2 wherein a slurry of an absorbant limestone material is circulated through the apparatus and a grinding mill in the circulation system removes the sulfite or sulfate compounds coating the particles of absorbant material so as to expose additional active surface.
Very recently, papers entitled "Pilot Plant Scrubber Test to Remove SO.sub.2 Using Soluble Alkali and Western Coal Flyash" by Tufte, Sondreal, Korpi, and Gronhovd, presented at the 1973 Lignite Symposium, Grand Forks, N.D., and "Some Studies on Stack Emissions from Lignite-Fire Power Plants" by Gronhovd, Tufte, and Selle, also presented at the 1973 Lignite Symposium, show that an available source of chemical reagent for removal of SO.sub.2 from a liquid scrubbing agent was found in the soluble-alkali flyash generated along with SO.sub.2 in the power plant stack emissions. In their "Pilot Plant Scrubber Test" paper, Tufte et al teach the use of two pilot wet scrubbers which use as the source of alkali, stack emission-derived flyash. The tests which recycled the emitted flyash to improve reaction showed that somewhat significant amounts of SO.sub.2 were removed from the SO.sub.2 -containing liquid scrubbing agent, particularly when the emission ash is high in alkali content. Though these studies show the possibility of the use of the alkali-associated flyash for removing SO.sub.2, the systems taught were not able to comparatively compete with that of a scrubber system using lime as the reaction reagent.
In commercial scrubbers now recycling emission flyash as described in the papers above, a greater percentage of the alkali constituents of the flyash is utilized by either recirculating the slurry through the scrubber or by mixing the slurry of collected flyash for an extended period of time to allow maximum reaction of the flyash-alkali constituents with SO.sub.2. Both of these methods have disadvantages. Specifically, when a slurry is recirculated through a scrubber, additional wear and constant plugging result. When holding the flyash in a tank and mixing for extended periods, equipment size and maintenance costs of the system become very great. An example of a commercial system using the first of these two approaches is the Dave Johnston Station of Pacific Power and Light Company at Glenrock, Wyo. This station currently recirculates flyash through their wet scrubbers on their 330 megawatt line. Accumulation of flyash materials in the scrubbers and appurtenances causes several shutdowns per week of scrubbing equipment and a resulting loss of generating capacity. In efforts to alleviate these problems, the Bureau of Mines has developed a mixed delay system according to the second of these two approaches. In the Bureau of Mines system, the scrubber discharge slurry is held and mixed for a period of about 55 minutes before the liquor is decanted from the slurry and returned to the scrubber for another gas-cleaning pass. For a typical 350 megawatt generator, this means that a mixing tank of over 1,000,000 gallons capacity is required (along with a great deal of mixing horsepower).